A Sense of Scale and The Motions of Earth. The guitar player Pablo Picasso (1910)

Transcription

1 A Sense of Scale and The Motions of Earth The guitar player Pablo Picasso (1910)

2 Announcements n Notes from the first lecture are available on the class web site ( n In-class quiz # 1 will take place Thursday, Sept. 15 th (next week): n It will include lectures 1 through 3, and the textbook units 1, 2, 3, 5, and 6 n It will be distributed towards the end of Thursday s class, around 10:30 pm; n You can consult notes/textbook n Please, remember to bring a hand-held calculator and a pencil.

3 Goals for Today n To get a sense of the scales of the Universe n To work on understanding one particular way of looking at how the night sky changes with time.

4 Reading Assignments n Units 3 and 5 of Textbook

5 Fundamental Astronomical Assumption: n Astronomers assume that the laws of physics and chemistry are the same throughout the Universe.

6 A Sense of Time If we were to compress the time since the Big Bang into one year, and make the time of the Big Bang January 1, v The Earth was formed in mid-september. v The mammals appeared on December 26. v All human prehistory (from the first known stone tools) and history have occurred in the last ½ hour of New Year's Eve. All of human history is but a fleeting instant on the cosmic timescale.

7 Special Astronomical Units Light-time=Light traveling time=distance Car-traveling time as distance: Boston is about 1.5 hours away (assuming the speed of a car is 60 mph) The speed of light: c = 3x10 5 km/s. Destination Distance (km) Light Travel time London 6, s Moon 385, s Sun 1.5 x s (8.3 min) Jupiter 7.8 x min Nearest Star 4 x years Most Distant Galaxy 1.4 x billion years Whenever you see "light-(time) (light-year, light-minute, lightsecond), that means we are talking about distance, not time.

8 A Sense of Space 1. The Sun would hold 1.3 million Earths. The radius of the Sun is about 100 times that of the Earth. 2. There are ~100 billion "Suns" in a galaxy like our own Milky Way Galaxy. 3.Astronomers can see billions of galaxies.

9 Basic classes of Objects Planets: Earth as one Stars Sun as one Galaxies Milky Way as one Their sizes: Earth: 10 4 km poppy seed Sun: 10 6 km grape fruit Milky Way km Universe km Earth-Sun distance 100,000 Earth-Sun distance

10 Survey Question: If the Sun were a grapefruit on one side of the room, what would be Earth? 1) A poppy seed on the other side. 2) Another grapefruit on the other side. 3) A poppy seed in my office. 4) An apple on the other side. (Earth-Sun distance~100x radius of Sun~ 100X100 radius of Earth)

11 Survey Question: If the Sun were a grapefruit on one side of the room, what would be Earth? 1) A poppy seed on the other side. 2) Another grapefruit on the other side. 3) A poppy seed in my office. 4) An apple on the other side.

12 Survey question: What would then the nearest star be? 1) A poppy seed at the Amherst downtown. 2) A poppy seed on the west coast 3) A grapefruit at the Amherst downtown. 4) A grapefruit on the west coast. (Nearest star is 4Ly ~ 300,000 AU)

13 Survey question: What would then the nearest star be? 1) A poppy seed at the Amherst downtown. 2) A poppy seed on the west coast 3) A grapefruit at the Amherst downtown. 4) A grapefruit on the west coast.

14 Another unit astronomers love: Parsec: 1 parsec = 3.26 lyr The closest star to us: Alpha Centauri: 4.2 lyrs = 1.3 pc Our own Galaxy: diameter= 75,000 lyrs = pc = 23 kpc The closest galaxy: Andromeda: distance = 700,000 pc = 0.7 Mpc The Universe: Size = 1.3 x cm = 13,500,000,000 lyrs = 4300 Mpc

15 Now on to some game of scales.

16 52 feet across

17 1 mile across

18 160 kilometers across 1 mile = 1.61 kilometers Infrared photo

19 Diameter = 12,756 km

20 1,600,000 km =1.6 x 10 6 km Moon

21 1AU = 1.5x10 11 m

22 1.6 x km, or a trillion (10 12 ) times wider than the first picture! 32 AU

23 100 AU

24 1,000 AU

25 10,000 AU

26 10 6 AU = 17 ly 1 ly =63,000 AU = km

27 1700 ly

28 170,000 ly

29 1.7 x 10 7 ly

30 1.7 x 10 9 ly

31 Your local address:

32 The Motions of Earth 1. The Earth rotates this leads to day and night. 2. The Earth revolves around (orbits) the Sun this leads to the seasons. 3. The Earth and Sun revolve around (orbit) the center of the Galaxy. 4. The Galaxy moves through the Universe.

33 Everything in the Universe is Moving! but n the further away from us something is, the more it has to move to be noticeable. n the motion of the Earth rotating on itself (day/ night) is the easiest to see n Followed by the motion of the Moon around the Earth (~29 days) n followed by the motion of the Earth around the Sun n and finally the motion (over tens or hundreds of years) of other stars

34 How Do We Locate Objects on the Sky? The Celestial Sphere (C.S.) Stars, planets, the Sun are at different distances from us, yet for studying patterns in the sky, we can imagine that all are on the C.S.

35 The patterns of stars on the C.S. can suggest shapes of animals, or cultural objects. Today, we call them constellations.

36 Constellations The first thing to remember is that the groupings of stars that most people call constellations are not actual objects! They are `maps for the sky. In fact, the original constellations were invented by farmers over 5000 years ago.

37 There are 88 official constellations recognized by the International Astronomical Union. The names of constellations are in Latin. But most bright star names derived from ancient Arabic. The original constellations were invented over 5000 years ago. Remember that the constellations are not real!

38 Projection Big Dipper ---- an asterism, i.e., not an actual constellation, but a recognizable pattern in the sky

39 Orienting Ourselves on the C.S.

40 There are two ways to think about this: - the C.S. rotates - or the Earth rotates while the sky sits still. Nadir Sometimes one works better than the other

41 Are there stars in the sky during daytime?

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43 Stars, like the Sun and the Moon, appear to rise to the east, and set to the west.

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45 Only the Celestial Poles do not show to move. Polaris The star called Polaris marks the position of the Celestial North Pole and the Earth s North Pole. Often called the `North Star.

46 Latitude and longitude make a convenient coordinate system for locating objects on the Earth. In the same way, Right Ascension (RA, divided in 24 hrs) and Declination (DEC, in degrees) give us the direction to look on the celestial sphere.

47 Altitude and Azimuth give us the direction to look on the celestial sphere. The altitude of Polaris above the horizon is approximately the same as the observer's latitude in the Northern Hemisphere.

48 Survey Question In this time-laps photo, the stars make stripes due to: 1) the Earth rotating. 2) the Earth revolving around the Sun. 3) the Earth and Sun revolving around the Galactic center. 4) the precession of the Earth s axis.

49 The Earth orbits around the Sun on a plane called the Ecliptic (or Ecliptic Plane)

50 Recall that last time we defined the Constellations as apparent Associations of stars on the Celestial Sphere Q: What s the difference between the constellations and the zodiac? A: The zodiac is made up of the constellations which lie in the ecliptic plane.

51 As the Earth orbits around the Sun, the Sun appears from the Earth to `cross the Constellations. It takes 1 year for the Sun to cross all constellations in the zodiac.

52 Survey Question What zodiacal constellation is highest in the sky at midnight in June? 1) Libra 2) Pisces 3) Gemini 4) Capricorn

53 Libra Pisces Gemini Capricorn

54 Survey Question What zodiacal constellation rises at midnight in June? 1) Libra 2) Pisces 3) Gemini 4) Capricorn

55 Libra Pisces Gemini Capricorn

56 Survey Question One evening at midnight, you observe Leo high in the southern sky. Virgo is to the east of Leo and Cancer is to the west. One month earlier, which of these constellations was high in the southern sky at midnight? 1) Leo 2) Virgo 3) Cancer

57 Leo Virgo Cancer